I have experienced the deletion of old posts and some new posts. It is the moderators decision if they find any post irrelevant or if it is something that can cause spamming in the forum they will delete it. Your reply from that post will be deleted as well because of post deletion. You should not worry about those deleted posts as most of them were harmful for this forum and its users.
As long as it is visible here in Bitcointalk it will be visible on the internet and it will show up in the search engine, so posts that have no substance or are considered spam and redundant should be deleted, so when it comes to reference on any subjects about Bitcoin and Cryptocurrency related subjects those topics and discussion will come out and readers will appreciate the flow of the conversion.
So always check your posts and be sure that its relevant to the topic and is not redundant to avoid deletion.